The present invention is directed to a method of synthesizing a musical sound and more specifically to a technique utilizing frequency modulation to provide for the temporal evolution of the spectral components of a musical sound.
Several types of musical synthesizers are well known in the art but thus far the synthesis of natural sounds has been elusive. In a typical type of analog synthesizer, a voltage controlled oscillator is driven by a waveform of the desired shape and frequency and then filtered and passed through an attenuator to provide the proper envelope to simulate a desired musical or natural sound. With the foregoing analog synthesizer which is of the subtractive type, there is, of course, no evolution in time of the various spectral components or partial frequencies of the final sound.
Synthesizers utilizing digital techniques have realized that to create a natural sound individual partial frequencies must be generated and combined. One type of organ is based on this principle where the several partial frequencies are added together and then given a common envelope function. The combinations of such frequencies are based, of course, on the principles established in Fourier analysis.
Yet another digital synthesis technique has been suggested by Jean-Claude Risset and Max V. Mathews, "Analysis of Musical Instrument Tones," Physics Today, vol. 22, no. 2, pp. 23-30 (1969). Risset established that the character of the temporal evolution or the evolution in time of the spectral components of a sound is of critical importance in the determination of timbre. In other words, Risset would suggest that to simulate a natural sound the amplitude of each harmonic should be individually controlled as a function of time. In addition, Risset suggested, at least for the production of the trumpet tones, that the attack envelope, that is, the initial envelope characteristic for the trumpet tone, has a distinctive characteristic in that during the attack and also decay portion of the sound, the energy of the various frequency components evolve in complicated ways.
To implement the Risset theory with known techniques requires a complex digital computer which individually simulates each frequency component. Thus, at the present time a real time music synthesizer of the digital type is not commercially feasible.
The variation of bandwidth with modulation index has been illustrated in an article by Murlan S. Corrington, "Variation of Bandwidth With Modulation Index in Frequency Modulation," Selected Papers on Frequency Modulation, edited by Klapper (Dover Publications, 1970). However, this is merely a theoretical study of frequency modulation.